In conditioning and refrigeration technologies, fans of the axial type are used when it is necessary to generate air flows characterized by a high flow-rate/head ratio, i.e., when large amounts of air have to be moved over short paths or have to pass through means which have a high load loss (such as for example a high-efficiency filtration system).
The operation of a free axial impeller, shown schematically in FIG. 1, generates two volumes A and B at different pressures, which are divided by the disk-like band C occupied by the impeller in its rotary motion.
This simplified diagram reveals the first inherent limitation of a generic axial fan, i.e., the parasitic recirculation of air which occurs on the entire perimeter of the disk generated by the rotation of the impeller.
One of the limitations shown by the use of axial fans for the distribution of cold air in enclosed spaces is the shape of the output air jet.
This is one of the reasons why axial fans are mainly used in suction mode, since the manner in which the output air jet is constituted is of limited importance for the application.
In practice, fans arranged so as to work by aspirating air from above to propel it downward generate an output air flow which has the shape of a widening conical spiral.
The geometric characteristics of the profile formed by said flow depend on the constructive characteristics of the impeller and on the rotation conditions.
Further, the presence of the motor centrally with respect to the impeller generates a sort of ascending central recirculation, i.e., back toward the impeller, which facilitates the conical widening of the descending air flow.
Flow straighteners are currently commercially known which are designed to be arranged downstream of the impellers when a flow which is as cylindrical as possible has to be obtained.
These straighteners can be of various kinds.
A first type of straightener consists of straighteners constituted by a grille with rather fine mesh and having a certain thickness.
Such grille-type straighteners have the advantage of being compact and the possible disadvantage of a high load loss.
A second type of straightener consists of straighteners constituted by a static set of vanes which cooperate with the vanes of the impeller of the axial fan with which they are associated.
This second type of straightener, although being better than the first one, can be very bulky, but most of all is generally very expensive, since the profile of the vanes must be studied and built accurately also according to the profile of the vanes of the impeller of the fan.
Further, the design effort for manufacturing these straighteners with static vanes leads to solutions with the same inherent problem as the fans whose flow is to be corrected, and are therefore optimized for certain rotation condition.
A conditioning unit to be installed on a ceiling, provided with at least one axial fan which is arranged so as to work by aspirating air from above in order to push it downward, likewise generates an output air flow which has a conical spiral shape as described above.
The problem to be solved for these conditioning units is therefore the orientation of the air flow which exits downward.
Considering the cone-like widening of the flow and the recirculation caused by the central position of the motor of the impeller, said conical opening reduces the penetration of cold air to the lower layers of the cooled volume and facilitates the mixing of the layers of air at different temperatures, reducing the stratification effect which is generally sought and desired for the controlled conditioning of an enclosed space.
For these conditioning units it is therefore essential to obtain an air flow which is as concentrated as possible, so that the entire flow generated by the axial fan reaches the lower layers of the cooled volume, reducing recirculations and improving cold distribution, balancing it especially horizontally and stratifying it vertically.
Another problem for these conditioning units with an axial fan with descending vertical action is said parasitic air recirculation, which occurs on the entire perimeter of the disk formed by the rotation of the impeller.
This recirculation in fact reduces the efficiency of said conditioning unit and consequently has a negative effect also on the other associated machines, such as remote condensing units, water cooling units with air condensation, and the like.